(1) Field of the Invention
The present invention relates to a system and a process for providing power using a semiclosed Brayton cycle with direct heat transfer. More particularly the invention relates to a diesel fueled Brayton cycle system and process using CO2 as a major portion of the working fluid. This system is of particular use in torpedo and unmanned underwater vehicle propulsion systems.
(2) Description of the Prior Art
Current underwater propulsion systems are typically closed Rankine cycle power systems utilizing lithium as a fuel, a chlorofluorocarbon as an oxidant, and water as a working fluid. In a Rankine system, the working fluid is compressed, heated until vaporization, and then expanded through a turbine to produce power. Upon exiting the turbine, the low pressure vapor is condensed to a liquid, and the cycle is repeated. In a typical underwater propulsion system the working fluid is heated as it passes through heat transfer tubes that are wrapped to form a cylindrical annulus within the system's heat exchanger. The center of the cylinder contains liquid metal fuel to heat the working fluid carried by the heat transfer tubes. The working fluid, water, and the liquid metal fuel, lithium, react chemically with one another; therefore, a leak in the heat transfer tubes causes a violent reaction which generates a significant amount of heat and gas resulting in failure of the heat exchanger and the underwater device. Furthermore, should a leak occur in a land based system, the system will release a toxic cloud of LiOH into the environment. Other problems associated with the Rankine cycle include noise generation during the phase change of the working fluid, severe stress of the oxidant injectors due to high reaction zone temperatures, and slow start up time.
An alternative to the closed cycle Rankine power system is a closed or semiclosed Brayton cycle system. In a Brayton cycle, there is no phase change and accordingly, no noise associated therewith. The Brayton cycle is also more efficient than the Rankine cycle despite the fact that more energy is required to compress a gas than to pump an equivalent mass of liquid. Underwater propulsion systems cannot use prior art Brayton cycle systems because the components of the Brayton cycle, principally the conventional Brayton heat exchanger, will not fit in the restricted space available in underwater vehicles.
A compact heat exchanger can be made by increasing gas velocity through the heat exchanger to achieve higher heat transfer coefficients; however, this results in greater heat exchanger pressure drop. Increasing gas velocity is used successfully in Rankine cycle systems because pump power is a small fraction of gross power (1/50) and pump losses are small by comparison. Accordingly, there is no significant reduction in cycle efficiency. In the Brayton cycle, however, compressor power is typically a large part of the gross power (1/2); therefore, small increases in gas velocity and heater pressure drop reduce the Brayton cycle efficiency below that of the Rankine cycle.
My other listed inventions with which this application is copending relate to direct contact closed Brayton cycle power systems using liquid metal fuel. The size and weight penalty of the Brayton's hot side heat exchanger is eliminated by direct contact heat transfer between the working fluid which is an inert gas such as helium, argon, xenon, or a mixture of inert gases, and a liquid metal bath of a material such as lithium, sodium, potassium, aluminum, magnesium, or an alloy.